The bus was en route to Karbala from Damascus, Syria, and was in the al-Nukhaib area when it was stopped by gunmen, said police officials in Ramadi, the provincial capital. The gunmen then separated the men on the bus -- including the Syrian driver -- from the women and children. They then took the men to another location and shot them, the officials said.

Shiite pilgrims from Karbala often travel by bus to Damascus to visit the Syeda Zainab shrine.

Al-Nukhaib is about 270 kilometers (168 miles) southwest of Baghdad.

Officials of al-Nukhaib's city council have been accusing the neighboring province of Karbala of trying to exert their influence over this vital overland gateway to Saudi Arabia.

Separately, in Baquba, a Sunni imam was shot and killed by gunmen Monday in the al-Gatoon area, an official with Iraq's interior ministry told CNN on condition of anonymity because he is not authorized to speak to media. Ahmed Mahmud al-Jabalawi, the imam of the al-Shuhada mosque, was on his way to the mosque for morning prayers when he was shot by gunmen who used pistols equipped with silencers, the official said.

Baquba is about 60 kilometers (37 miles) north of Baghdad.

The official with the interior ministry gave details of other violence on Monday:

-- Six Iraqi soldiers were wounded when attackers threw two grenades into an Iraqi army patrol in al-Hadba neighborhood in central Mosul, the interior ministry spokesman said.

-- In Mansour neighborhood in western Baghdad, three roadside bombs exploded outside a liquor store and wounded two people.

-- In al-Iskndariya, two people were wounded when a roadside bomb exploded at a civilian car. Al-Iskndariya is about 50 kilometers (31 miles) south of Baghdad.